Extremely slow Read/Write SanDisk 400GB Extreme microSDXC UHS-I/U3 A2 Memory Card

Is this eligible for RMA?

Bought from Newegg.ca 3/15/2019.

I have been using this in my Nintendo Switch and only recently noticed some terribly long loading times.

Took it out and ran it through HWTEST and am getting an abyssmal 10MB/s READ !

Tried it with two different USB 3.0 SD Readers. In Windows 7 and 10. Same speeds throughout!

Tested my 256GB Ultra A1 and get 40-60MB/s same Readers, so its not bad readers (albeit those speeds are “low” for 100MB/s rated card)

In case it matters, Formated as exFAT.

Is there anything extra I can try to fix this? I have already backed up all the data to PC (took 1.2 DAYS!) so not afraid to format or send in for RMA if I am still eligible. 

Having similar issues with 1TB Sandisk Extreme MicroSDCS UHS-1 (not on Nintendo Switch though).  Used HWTest for metrics in a Surface Pro 5 card in device slot and using Sandisk USB-A adapter write speed degraded over time to the point that it was never going to finish the full 1TB test. 

Edit: formatted from exfat32 to NTFS made no difference

Ran some more tests and I think the problem is with how Widows 10 manages H2TestW.  I think what’s happening is Windows is constraining the preformance of application to last for the remaining battery life of the Surface Pro.  When I just try to transfer 900 GB of data from a network drive to the MicroSD card the write speed is relatively constant at around 65MB/s.  Try running the test with PowerMode set to Best performance and your device not running on battery power.

Same with my 200GB card with a Switch Lite.  Getting 15mb/s read, 8mb/s write.  Reverted back to a 64GB card (45mb/s read. 16 mb/s write).  I want to know if I can RMA also but it was a Christmas gift.  Panzer Dragoon was taking almost 5-minutes to load levels!

I’ve been having the same problem with my Sandisk Extreme USB 3.0. Superfast then drops down to like 4.7 MB/s or even KB/s then jumps up to 25 MB/s on repeat.
When I got it, the speeds were in the 70s and higher then after a couple of weeks this started happening. Been a few months now.

Someone said the following methods can promote the read/write speed of your SD card. But I’ve tried this to no avail. Maybe it will work for you! Open Device Manager and select Disk Drives. Find your thumb drive in the list and right click on it. Select Properties and then Policies. Finally, Select “Better Performance”.

You can enchance the performance by doing Full format on windows, go here >> How to remove write penalty